Plants on a green roof, whether they were intentionally planted or not, are an
indicator for the performance of any green roof system. If any extensive
green roof system becomes high maintenance, requiring irrigation or
extra nutrients, the whole purpose of the green roof is defeated.
Understanding these complex synergetic effects takes decades of
experience in the field, the patience of gardeners and common sense.

Jörg's blog regarding
irrigation on extensive green roofs has recently been published on
greenroofs.com. An extended version of the original, Jörg goes more in
depth on why irrigation on green roofs should not be necessary.